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What is Sonoluminescence, and Why Don’t We Know How it Works


What is Sonoluminescence, and Why Don’t We Know How it Works

It’s 2013 and we as humans are creating everything from organic machines to robots that twenty years ago were only a part of science fiction. We have computers more powerful than the NASA computer used to send the shuttle to the moon in 1963, and we carry them around in our pockets. Despite all of this, Mother Nature still has her mysteries, including the effect of Sonoluminescence. What is it? Quite simply, sonoluminescence is the light created when you burst an underwater bubble with sound.


sonoluminescenceSonoluminescence was first discovered in 1934 as part of a study in sonar at the University of Cologne. H. Frenzel and H. Schultes had placed an ultrasound transducer which emits high frequency sound into a tank filled with developing fluid for photographs in hopes of developing the photos more quickly. Instead, they were amazed to see dots on their film, which was a result of the flashes of light caused by the sonoluminescence. Since then, the effect has been studied consistently, with major research in 1989, and now. In 1989 Felipe Gaitan and Lawrence Crum created a stable method of testing sonoluminescence by creating a bubble that creates the flash of light as it decompresses rather than pops, which allows for easier testing. This is called SBSL (Single Bubble Sonoluminescence).

What is Sonoluminescence?

Sonoluminescence is, as stated above, the light created when you burst an underwater bubble using sound. Most bubbles are made of gasses, and the sound must be of sufficient intensity to burst the bubble, although the frequencies required do vary depending on the height, thickness, and even the gas present inside of the bubble.

When the bubble shrinks to smaller than a micrometer, it emits a light, which is usually only visible for 35-200 picoseconds of time. Because a picosecond is one trillionth of a second, that period of time is very short, but the light still exists, and we don’t know why. Some of the things we do know is that noble gasses such as helium, argon, or xenon emit more light, and that it is possible to set up ‘stable’ bubbles and consistently pop them to create re-occurring light. The average bubble reaches 2,300 to 5,100 kelvin, but some rare bubbles have been recorded with light burst temperatures of 20,000 kelvin, and electric charge of some 18 volts.



So what theories do we have about sonoluminescence? Actually quite a few, while we don’t know enough to prove these theories, any one of them could be true. Some of the theories include hotspot, bremsstrahlung radiation, collision-induced radiation and corona discharges, nonclassical light, proton tunneling, electrodynamic jets and fractoluminescent jets as well as quantum physics and even nuclear reaction theories!

sonoluminescenceThe mechanism of the sonoluminescence shows that the bubble rapidly collapses and the pressure from the water and the collapse often causes the bubble to reach temperatures of upwards of 10,000 kelvin. This ionizes the fraction of gas, especially noble gas, present in the bubble, which allows the light to be visible. The thermal bremsstrahlung radiation which so far is one of the most popular theories, shows that ionized atoms would react with neutral atoms to cause radiation, which could be the cause of the light.

Studies have shown that everything about the bubble can be completely different and the light will still appear. Bubbles at different temperatures, in different solutions (sulfur, water, solutions of noble gasses, etc.), with different thicknesses, sizes, and chemistry all create light. Some create more light, some are hotter than others. Notably bubbles with nitrogen or oxygen do not emit light, and we don’t know why not either.

While there are many theories to explain sonoluminescence, there are no proven facts. So far, this amazing burst of light is just another of Mother Nature’s mysteries, and looks to remain that way for at least a few years.

Want to see Sonoluminescence in action? Check out this video and keep in mind that the flashes are small, but highly visible.



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